Transition metal borides have found a number of technologically important applications, among which the most notable is their use as permanent magnets based on neodymium iron boride, Nd2Fe14B. See J. F. Herbst, Rev. Mod. Phys., 63 (1991) 819-898. The research on the magnetism of complex intermetallic borides thus has been predominantly focused on the rare-earth containing systems with strong magnetic anisotropy. The latter, when combined with the high saturation magnetization of the transition metal sublattice, offers the highest energy products and thus the strongest permanent magnets known. See O. Gutfleisch, M. A. Willard, E. Bruck, C. H. Chen, S. G. Sankar, J. P. Liu, Adv. Mater., 23 (2011) 821-842. In contrast, the magnetism of rare-earth free borides is far less explored. Such materials usually behave as soft magnets, which could be one of the reasons why their magnetic behavior has not inspired as much research interest as the properties of the rare-earth containing borides. Nevertheless, two recent thrusts poise rare-earth free magnetic materials to gain increased attention. The first is the need to discover novel permanent magnets with decreased rare-earth content. See Critical Materials Strategy, U.S. Department of Energy, Washington, D.C., 2010. The second direction is due to the discovery of giant magnetocaloric effect at room temperature that promises to become the foundation of the future refrigeration technology. See K. A. Gschneidner, Jr., V. K. Pecharsky, A. O. Tsokol, Rep. Prog. Phys., 68 (2005) 1479-1539; B. G. Shen, J. R. Sun, F. X. Hu, H. W. Zhang, Z. H. Cheng, Adv. Mater., 21 (2009) 4545-4564; and V. Franco, J. S. Blazquez, B. Ingale, A. Conde, Annu. Rev. Mater. Res., 42 (2012) 305-342. The latter requires the use of soft magnets with high saturation magnetization to achieve a large cooling effect while avoiding hysteretic energy losses in a quickly alternating magnetic field.
We have recently reported the promising magnetocaloric properties of AlFe2B2, a ternary boride with a rather simple layered structure, the magnetic behavior of which went overlooked for more than 40 years. See X. Y. Tan, P. Chai, C. M. Thompson, M. Shatruk, J. Am. Chem. Soc., 135 (2013) 9553-9557 and W. Jeitschko, Acta Crystallogr. Sect. B, 25 (1969) 163-165. Our initial interest in this material was sparked by the high saturation magnetization offered by FeB. The ordering temperature of this ferromagnet, however, is too high for practical purposes (around 600 K). Consequently, we turned to the ternary material that affords a “diluted” magnetic lattice featuring two-dimensional (2-D) [Fe2B2] slabs alternating with layers of Al atoms along the b axis of the orthorhombic unit cell. See FIG. 1, which is a depiction of the crystal structures of AlFe2B2. The [Fe2B2] slabs are highlighted (Fe=larger atoms and B=smaller atoms in the highlighted slabs). Al atoms are located between the [Fe2B2] slabs. AlFe2B2 shows ferromagnetic ordering at ˜300 K nearly zero coercivity, and a significant magnetocaloric effect. Another attractive feature of this material is its being composed of earth-abundant, lightweight elements.